The present invention relates generally to the problem of aluminum alloy workpieces absorbing hydrogen when undergoing heat treatment in furnaces containing ambient moisture-laden atmospheres, and particularly to a low molecular weight alkyl, alkylene or aryl phosphonic acid treatment that substantially reduces the absorption of hydrogen into aluminum alloy workpieces and, in addition, greatly enhances hydrogen degassing of such workpieces.
During the fabrication of aluminum alloy products, various types of heat treatments are used to: improve aluminum formability, improve compositional uniformity, relieve stresses and/or improve mechanical properties. The maximum aluminum alloy soak temperature varies with the alloy and type of heat treatment. These maximum metal soak temperatures typically fall within the range of about 454.degree. C.-635.degree. C. during preheats, within about 315.degree. C.-471.degree. C. during reheats, within about 315.degree. C.-413.degree. C. during anneals and within about 443.degree. C.-552.degree. C. during solution heat treatments. During these heat treatments, the protective oxide layer on the aluminum alloy workpieces is invariably disrupted to expose nascent aluminum. The exposed aluminum can undergo high temperature oxidation reactions with water producing oxidized aluminum phases and atomic hydrogen. Atomic hydrogen is the only gas that has appreciable solubility in solid aluminum and can readily diffuse into the aluminum object. Still, adsorbed atomic hydrogen has limited solubility and the propensity to precipitate as insoluble molecular hydrogen (H.sub.2) at heterogeneities or defects, especially in the more highly deformed areas of an aluminum workpiece. As increasing molecular hydrogen is precipitated within the metal, additional atomic hydrogen can diffuse inward and be accommodated within the metal matrix. Precipitated molecular hydrogen forms secondary porosity, which may compromise the structural integrity and mechanical performance of the final aluminum part.
The movement of atomic hydrogen in aluminum alloy lattice with a given hydrogen concentration gradient is described quantitatively in terms of a diffusion coefficient. The hydrogen diffusion coefficient varies with the composition and history of the aluminum alloy workpiece, but always increases exponentially with increasing temperature. Thus, at room temperature, the diffusivity of atomic hydrogen in aluminum alloys is insignificant and the hydrogen contents of workpieces made therefrom will not change appreciably over time. At elevated heat treatment temperatures typically used of greater than 300.degree. C., however, the diffusivity of hydrogen can play a dichotomous role in the control of hydrogen in aluminum alloy workpieces. If the atomic hydrogen concentration is greater at the surface of the aluminum workpiece than within the workpiece, hydrogen diffusion will progress inward and the bulk hydrogen content will increase. Thus, the increased diffusivity at elevated furnace temperatures above 300.degree. C. can result in significant hydrogen accumulation in aluminum alloy workpieces during heat treatments, ultimately originating from water or water vapor in ambient furnace atmospheres. If, however, the atomic hydrogen concentration at the aluminum surface is less than within the bulk, hydrogen diffusion will progress outward and the bulk hydrogen content will decrease. Thus, the increased diffusivity at elevated furnace temperatures above 300.degree. C. provides the opportunity to facilitate significant reduction of hydrogen already accumulated during casting and previous heat treatment processing.
For several decades, ammonium fluoborate (NH.sub.4 BF.sub.4) protective atmospheres have been used in the industry to prevent substantial absorption of hydrogen by aluminum alloy workpieces during high temperature furnace treatments in the presence of moist air. Ammonium fluoborate decomposes at temperatures above 250.degree. C. (during the initial heat ramp up to the maximum soak temperature) to form a blanket atmosphere that fills the entire internal volume of a furnace. Ammonium fluoborate also produces an array of compounds in the furnace which can eliminate high temperature oxidation reactions by either reacting with ambient water or by forming a protective fluorinated layer on the aluminum alloy workpiece.
There are drawbacks to the use of ammonium fluoborate atmospheres, however. Ammonium fluoborate species can stain and pit surfaces of some aluminum alloys. The ammonium fluoborate decomposition products contain toxic, corrosive and particulate species. The ammonium fluoborate emissions corrode furnace structures and baghouses for filtering particulate emissions. Disposal of the collected particulates is costly. Concerns relating to the emissions have prompted research to identify alternative chemistries that are more environmentally friendly and safer for in-plant use.